The present invention relates to a device for compressing binary signals, particularly intended to be incorporated in a system for coded facsimile transmission of documents with the object of reducing the transmission time by reducing the length of the series of binary signals associated with the sequentially scanned lines of picture elements of the document to be transmitted.
The invention also relates to all coded facsimile document transmission systems having such a device for compressing binary signals.
A statistical study shows that in most documents to be transmitted the white characters are in the majority, the black information bearing characters being in the minority (in the further course of this description the characters will always be referred to as being white or black, but it must be understood that the invention is also applicable in those cases where the characters which constitute the information to be transmitted and the carrier of these characters are initially available in other colours). If a reduction in the transmission time is required, it is very important to compress the character areas which are met when scanning the picture constituted by the document. This compression can be obtained by means of a suitable binary coding of the picture elements of each scanned line of the document to be transmitted, using the horizontal correlation in each of these lines of picture elements; but the use of one-dimensional codes (such as, for example, the normalized EIA-BFICC code, recommended by the Comit Consultatif International de Telephonie et Telegrahie) does not result in very high compression rates.
A greater reduction in the transmission time is obtained when the correlations between consecutive lines of the documents is used in combination with two-dimensional codes. The so-called sequential codes are, for example, used in which each line is coded with respect to the preceding line; to limit the risk of the propagation of errors, which are inherent in this coding method, one line out of k lines can be transmitted in a one-dimensional code. This sequential coding can for example be obtained by "predicting" the value of each considered picture element of the line, based on the picture elements of the line predicted at an earlier instant and being adjacent to the considered element and, possibly, based on the picture elements predicted at an earlier instant and being in the same line.
Predictive coding techniques have already been described, for example in the periodical "IBM Journal of Research and Development"(March 1974), Image Data Compression by Predictive Coding: Prediction and Encoding Algorithms, by: L. R. Bahl and H. Kobayashi. However, whatever the number of picture elements used in the predictors suggested in this periodical, the results of the prediction are mediocre in the areas of the document having many transitions. The quality of the prediction can be improved by increasing the number of picture elements belonging to the predictor, but this increase entails an exponential growth of the dimensions of the decision table and therefore of the associated store.